In Journal of the America Chemical Society 115 (1993) 6382, Gratzel and others reported a dye sensitized photoelectric conversion element (Gratzel cell). This cell was made by forming a film of an organic dye which had a photoelectric conversion function on a transparent electrode, such as a titanium oxide, followed by filling between electrodes with an electrolyte. It was reported that this cell exhibited a similar performance to an amorphous silicon photoelectric conversion element.
However, since this dye sensitized photoelectric conversion element is a wet solar cell which performs electrical junction with a counter electrode with a liquid redox electrolyte, if it is used for a long period of time, a photoelectric conversion function will fall remarkably by exhaustion of an electrolyte, and consequently, it may stop functioning as a photoelectric conversion element.
Then, as a photoelectric conversion element which uses no electrolyte and which can be prepared by a cost saving solution coating method, it is proposed an organic photoelectric conversion element equipped with a bulk heterojunction layer, for example, in Patent document 1. This is a heterojunction (laminated type) element which sandwiches an electron donor layer (a p-type semiconductor layer) and an electron acceptor layer (an n-type semiconductor layer) between a transparent electrode and a counter electrode, or an element which sandwiches a p-type semiconductor polymer and an n-type semiconductor material between a transparent electrode and a counter electrode.
The operation principle of these photoelectric conversion elements will be described. First, an exciton generated by photoexcitation moves from a p-type semiconductor layer to an interface of an n-type semiconductor layer, and this exciton supplies an electron acceptor layer with an electron. Thereby, a positive hole is generated in the p-type semiconductor layer, and an electron is generated in the n-type semiconductor layer. And the positive hole is carried to one electrode through the p-type semiconductor layer by an internal electric field, and the electron is carried to another electrode through the n-type semiconductor layer. As a result, a photocurrent is observed.
However, in the heterojunction type photoelectric conversion element, unlike a Si type photoelectric conversion element, since the lifetime of the positive hole and the electron is short and their mobility is low, the positive hole and the electron generated will be deactivated before reaching an anode and a cathode, respectively. And there is a problem that an electric charge cannot be taken out to result in failing to improve photoelectric conversion efficiency.
A non-patent document 1 is cited as a literature which analyzed the reason of this effect. In this literature, it is reported that the maximum photoelectric conversion efficiency is obtained in the portion where the mobility of a p-type semiconductor layer and an n-type semiconductor layer is almost equal in a bulk heterojunction type organic photoelectric conversion element.
Generally, as an organic semiconductor material, it is known that mobility of a low molecular weight material is higher than a polymer material. From a viewpoint of mobility, it is advantageous to use a low molecular weight material for both a p-type semiconductor material and an n-type semiconductor material. There is a report which uses a low molecular weight material for both a p-type semiconductor material and an n-type semiconductor material (Patent document 2), however, when combination of low molecular weight materials is used for preparing a coated organic photoelectric conversion element, one of the materials will take a discontinuous structure, and formation of a carrier path which transmits a positive hole or an electron to an electrode is difficult, and a high efficient organic photoelectric conversion element has not been not obtained.
On the other hand, an approach of balancing the mobility of a p-type semiconductor material and an n-type semiconductor material is also disclosed by making both a p-type semiconductor material and an n-type semiconductor material a polymer (Patent document 3). However, when both of them are polymer materials, both of mobility will fall and, high photoelectric conversion efficiency has not been acquired.
Moreover, it is also expected to improve its lifetime of the organic bulk heterojunction photoelectric conversion element. In the process in which the carriers generated by receiving light are deactivated without reaching an electrode by recombination, deterioration of materials or increase of a carrier trap site occurs, and it is thought that it will cause deterioration of efficiency. Then, it is thought that it is required to improve the mobility simultaneously while the mobility of a p-type semiconductor layer and an n-type semiconductor layer are kept almost equivalent in order to achieve a higher efficiency and a longer lifetime.